1. Field of the Invention
The present invention relates to the field of earth-boring tools, and more particularly to a core barrel and a coring tool.
2. Description of the Prior Art
A coring tool is used to cut cylindrical cores from a rock formation, which cores are then retrieved for later laboratory analysis and examination. Thus, a coring tool generally comprises a coring bit which cuts a cylindrical core from the rock formation. As the core is cut, it moves within an inner gage of the coring bit, and is disposed within the outer tube or drill collars which are coupled to the coring bit. The drill collars are in turn coupled to drill pipe which constitutes the outer portion of the drill string and extends to the well surface. Within the outer tube of the coring tool is one or more sections of an inner tube assembly, including a core catcher disposed at the lower end of an inner tube shoe assembly. The core catcher is a device which engages the core through an interference fit and, when the coring tool has an upward tension placed thereon, jams between the core and the inner tube shoe assembly in order to seize the core, break it from the rock formation, and to retain it within the coring tool for retrieval at the well surface.
However, the inner tube assembly as manufactured and assembled in combination with various core bit designs does not have an exact length, but is characterized by a length which varies within a certain range. This range determines then the amount of spacing between the lower end of the inner tube assembly and the coring bit. This displacement is important in establishing the hydraulic communication from the interior of the drill string to the inner gage of the coring bit, and in providing a smooth transition between the inner gage of the coring bit and the lower end of the inner tube in order to minimize the disturbance of the core as the inner tube slides along the length of the core.
In order to accommodate these variations, the prior art practice in the field is to dispose a plurality of shims or washers within the coring tool in order to provide fine adjustments between the inner tube and coring bit. Generally, this requires disassembly of the cartridge cap from the safety pin in order to insert the shims or washers. This disassembly and assembly necessarily entails a certain amount of rig downtime during which all drilling operations are suspended.
Therefore, what is needed is some means whereby the longitudinal adjustment of an inner tube assembly within a coring tool can be quickly and easily made so that the loss of expensive rig time in the field can be avoided.